In recent years, a serious view has been taken for saving resources and cleaning up the environment, and thus, the reduction of fuel consumption of automobiles is required severely. For the reduction of fuel consumption, reduction of rolling resistance of tires is desired. The reduction of rolling resistance of tires is attained usually by using a rubber material capable of providing a rubber vulcanizate exhibiting a reduced heat build-up.
Incorporation of silica as a reinforcing agent instead of carbon black, in a diene rubber, has heretofore been proposed to give a rubber composition capable of exhibiting a reduced heat build-up. However, the silica-incorporated diene rubber composition has a problem such that the tensile strength and abrasion resistance are poor as compared with the carbon black-incorporated diene rubber composition. This would be due to the fact that silica has poorer affinity than carbon black to a diene rubber, and thus, the silica does not exhibit highly enhanced reinforcing effect.
Proposals of using a diene rubber having introduced therein a substituent exhibiting an improved affinity to the diene rubber have heretofore been made to enhance the affinity of silica to the diene rubber. For example, a diene rubber having introduced therein a tertiary amino group, which is prepared by an emulsion polymerization procedure (Japanese Unexamined Patent Publication (abbreviated to "JP-A") H1-101344), and a diene rubber having introduced therein an alkylsilyl group (JP-A H1-188501), a halogenated silyl group (JP-A H5-230286) or an amino group having a substituent or substituents (JP-A S64-22940), which are prepared by an anion polymerization procedure, were proposed. However, the diene rubbers having incorporated therein these substituents exhibit heat build-up, tensile strength and abrasion resistance, which are not improved to the desired extent.
A rubber composition exhibiting an enhanced green strength was proposed in Japanese Examined Patent Publication (abbreviated to "JP-B") S58-41282 wherein a rubber composition is disclosed, which comprises a product prepared by reacting a butadiene-styrene copolymer having a tertiary amino group in the main chain with a dihalide such as 4,4'-bis(bromoacetyl)diphenylmethane (hereinafter abbreviated to "BADM"), and carbon black. If this product is incorporated with silica, the tensile strength is improved to a certain extent, but, the effect of reducing the heat build-up, intended by the incorporation of silica, is not sufficient and the abrasion resistance and other properties also are not high.